1. Field of the Invention
This invention relates to compositions and methods for fabricating asymmetric fluoropolymer membranes comprised of fluoropolymer materials. This invention also relates to a method for separating one or more organic compounds from a mixture of organic compounds or one or more organic compounds from water by pervaporation or vapor permeation utilizing asymmetric fluoropolymer membranes of the present invention.
2. Background of the Invention
Pervaporation and vapor permeation are membrane processes used to separate mixtures of dissolved organic solvents and are extremely important in many industrial applications, e.g., the removal of organic pollutants from waste water. In the pervaporation process, a liquid mixture feed solution contacts the upstream side of a pervaporation (PV) membrane and the permeate is removed as a vapor on the downstream side. Transport through the membrane is achieved by the difference in partial pressure between the liquid feed solution and permeate vapor. Solvent/solute separation is achieved due to the difference in relative volatilities and membrane permeabilities of the feed solution compounds. The efficiency of a membrane pervaporation process is measured by its Flux and Separation Factor. Flux is a measure of the weight of solute which passes through the membrane per the membrane area per a unit of time. Separation Factor is the concentration ratio of solute to solvent in the permeate divided by the concentration ratio of solute to solvent in the feed solution.
The key element in a pervaporation separation process is the membrane itself. The critical physical and operational characteristics of a pervaporation membrane are that it exhibit:
1. high separation factor (high solute selectivity); PA1 2. high solute flux; PA1 3. chemical stability to a wide range of organic solutes and solvents; PA1 4. mechanical stability; and PA1 5. thermal stability. PA1 (a) dissolving a fluoropolymer material in a solvent to form a solution; PA1 (b) depositing the solution on a casting surface; and PA1 (c) removing the solvent from the solution, thereby precipitating the membrane therefrom. Any fluoropolymer material or fluorinated copolymer can be utilized in the method, including but not limited to, material selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, and polyvinylchlorotrifluoroethylene. PA1 (a) placing an asymmetric fluoropolymer membrane comprised of a fluoropolymer material into a suitable pervaporation or vapor permeation apparatus such that the membrane forms a selectively permeable barrier to one or more compounds of the mixture, the membrane having a first surface comprised of a dense layer of the fluoropolymer material, and an opposite second surface comprised of a porous layer of the fluoropolymer material; PA1 (b) contacting the first surface of the membrane with the mixture; and PA1 (c) transporting permeate through the membrane by creating a difference in the partial pressure of the compounds between the first and second surfaces of the membrane.
The method for fabricating the membranes should provide an easy means for altering the membrane microstructure, thereby changing membrane permeability and selectivity. Prior to this invention, however, attempts to fabricate a pervaporation membrane which exhibits all of these critical characteristics have been largely unsuccessful.
Many studies have been performed on the use of pervaporation membrane processes to separate azeotropic mixtures, e.g. alcohol and water (P. Aptel, N. Challard, J. Cuny, and J. Neel, Journal of Membrane Science, 1:271 (1976)) to dehydrate organic liquids (W. H. Schneider, "Purification of Anhydrous Organic Mixtures by Pervaporation," in Proceedings of the Second International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish, ed., pp. 169-175, (1987)), and to remove organics from groundwaters and industrial wastewaters (H. Eustache and G. Histi, Journal of Membrane Science, 8:105 (1981)); (T. Q. Nguyen and K. Nobe, Journal of Membrane Science, 30:11 (1987)); (J. P. Brun, C. Larchet, G. Bulvestre, and B. Auclair, Journal of Membrane Science, 25:55 (1985)). Much of the prior work on the separation of organics from water has focused on the use of PV membranes composed of elastomeric (rubbery) polymers. The most widely used elastomeric membrane materials are silicone rubber (polydimethylsiloxane) and copolymers thereof, followed by copolymers of styrene and styrene derivatives (R. Y. M. Huang, Ed., Pervaporation Membrane Separation Processes, Elsevier, pp 439 (1991)). Other membrane materials include soft segment elastomers such as polyether block amides (G. Bengtson and K. W. Boddeker, "Pervaporation of Low Volatiles from Water," in Proc. 3rd Int. Conference on Pervaporation Processes in the Chemical Industry, R. Bakish, ed., Englewood, N.J., pp. 439-448 (1987)), nitrile butadiene and styrene-butadiene rubbers (Brun et al. 1985)) and low density polyethylene (Eustache et al. (1981)). These membrane materials exhibit relatively high selectivity for removing different organic components like chloroform, benzene, toluene, ethanol, and acetone from dilute aqueous feed solutions. However, the organic fluxes are often low due to the fact that the membranes are thick and have a homogeneous (symmetric) microstructure. Likewise, these membranes are not very resistant to chemical and thermal degradation.
In an effort to improve organic transport rates, composite membranes were developed, wherein a thin permselective polymer top layer is coated on a sublayer which is microporous and offers little or no resistance against the permeating organic species. The top layer is usually composed of a different polymer material than the support layer. Examples of composite-coated PV membranes are the Code-100 and Code-200 silicone rubber membranes manufactured by Membrane Technology and Research, Inc. of Menlo Park, Calif. (J. G. Wijmans, J. Kaschemekat, J. E. Davidson, and R. W. Baker, "Treatment of Organic-Contaminated Wastewater Streams by Pervaporation," Environmental Progress, 9:262 (1990)). These membranes, however, require complex fabrication procedures and lack the desired resistance to chemical and thermal degradation. Therefore, there exists a need to develop a better pervaporation membrane with higher flux and separation factor than is seen with previous pervaporation membranes. There also exists a need for a pervaporation membrane fabricated: from more stable polymers, i.e., a PV membrane more resistant to chemical and thermal degradation.
As an alternative to homogeneous and composite-coated PV membranes, the one component asymmetric membrane has been tried for pervaporation separation. Such membranes are prepared by a phase inversion technique where a homogeneous polymer solution is cast as a thin film or spun as a hollow fiber and immersed in a non-solvent bath. Dense and microporous layers are formed from the single polymer by a combination of demixing of the casting solution, solvent removal, and precipitation of the polymer. A classic and well-known example of a commercially available phase-inversion membrane is the asymmetric cellulose acetate reverse-osmosis membrane (U.S. Pat. No. 3,133,132 (1964)). These membranes, however, proved to be ineffective in separating water/organic liquids in a pervaporation process (Huang et al. (1991). To date, researchers have been unsuccessful in fabricating new phase-inversion membranes for pervaporation separations.
Prior work on fluoropolymer pervaporation membranes has been quite limited. In 1972 McCandless reported on the use of polyvinylidene fluoride (PVDF) resin membranes which were plasticized with 3-methylsulfolene (F. P. McCandless, Ind. Eng. Chem. Process Des. Develop., 12:354 (1973)). These membranes were able to separate a variety of aromatic/naphthene organic mixtures (e.g., benzene/cyclohexane) but the aromatic separation factors were low (2.29-5.45). More recently, Belfort and co-workers used symmetric polyvinylidene fluoride PV membranes to separate ethanol and chloroform from water. For ethanol/water separation the performance of these PVDF membranes was comparable to silicone membranes (Y. M. Lee, D. Bourgeois, and G. Belfort, "Selective Organic Transport Through Polyvinylidene Fluoride (PVDF) for Pervaporation," in Proceedings of the Second International Conference on Pervaporation Processes in the Chemical Industry, R. Bakish, ed., pp. 58-70 (1987)). For chloroform separation, however, large permeation fluxes with separation factors .significantly lower than silicone characterized these PVDF membranes. Consequently, there still exists a need to provide a stable efficient and effective membrane for pervaporation separation.
The present invention satisfies that need by providing a composition and a method for fabricating an asymmetric pervaporation membrane comprised of a fluorinated polymeric material which exhibits improved performance over prior art membranes. The method for fabricating the membrane is unique and it is this method which imparts the asymmetric structure to the membrane. This method also provides an easy means for altering the membrane microstructure thereby changing membrane permeability and selectivity.
The membranes of the present invention posses separation and flux characteristics which make them highly suitable for use in organic/water and organic/organic pervaporation or vapor permeation separations, e.g., the removal of nonpolar organic pollutants from :wastewater (the removal of benzene, toluene, and chloroform from water), the separation of water soluble organics from water (e.g. dioxane, acetone, alcohols, and ethyl acetate), and the separation of mixtures of two or more organic components. The membranes are composed of a hydrophobic fluorinated hydrocarbon polymer which is highly inert to chemical attack by solvent or solute species. The membranes of the present invention can also be used at higher temperatures than traditional PV films such as silicone rubber membranes which have a temperature limit of approximately 60.degree. C., depending upon the extent of cross-linking.
The present invention also satisfies many industrial needs for an efficient and effective pervaporation process by providing a method for separation one or more organic compounds dissolved in a solvent of a liquid feed solution by pervaporation or vapor permeation utilizing the asymmetric fluoropolymer membranes of the invention.